1. Field of the Invention
The present invention relates to a light-emitting device comprising a plurality of cells or blocks integrally formed on a single substrate and each having a light emitting element and a transfer element having a self-scanning function and, more particularly, to a light-emitting device which realizes a long life and therefore can be applied to an optical printer or the like.
2. Description of the Prior Art
An LED (Light-Emitting Diode) and an LD (Laser Diode) are known as typical light-emitting elements.
In the LED, a PN or PIN junction made of a compound semiconductor (e.g , GaAs, GaP, AlGaAs, InGaAsP, or InGaAlAs) is formed. When a forward voltage is applied to the junction to inject carriers into the junction, the LED emits light during recombination.
The LD has a structure in which a waveguide is formed inside the LED.
As the light-emitting element, a negative resistive element (e.g., a light-emitting thyristor or a laser thyristor) having a light emission function is known. The light-emitting thyristor forms a pnpn structure on the compound semiconductor.
A large number of the LEDs are formed on a compound semiconductor substrate or wafer and cut into individual chips, or strips including aligned LED cells. When the chips or strips are applied as a light source for a LED printer, a read point and a write point of address to a medium such as paper must be are designated in the array. For this reason, a function of scanning light emission points with the light-emitting elements is required.
In order to perform optical scanning by using these conventional light-emitting elements, a plurality of the LED chips is soldered on the base with a predetermined pitch in alignment with the scanning direction to provide an LED array and each LED must be connected to a driving IC by a technique such as wire bonding so that the IC drives the LED. Therefore, if the number of LEDs is large, the same large number of wire bonding are required which increase manufacturing cost. As a result, a wide area for installing the driving ICs is required which makes it difficult to form a compact device. In addition, since the pitch between the ICs is determined by the wire bonding technique, it is difficult to arrange the LEDs with a small pitch.
The present inventors applied a self-scanning function to a light-emitting array as shown in FIGS. 1 to 9 to overcome the above problems (U.S. patent application Ser. No. 324,197).
The light-emitting array uses a light emitting element which can control a turn-on threshold level of the adjacent light-emitting element. Transfer of light emission in the scanning direction of the array is realized on the basis of a principle in that a change in turn-on threshold level caused upon turn-on of one element is propagates to the next element in the scanning direction. Therefore, at least one element in the array is always turned on to emit light. At this time, although a light emission amount can be controlled, a current capable of maintaining the ON state must be supplied to the element.
That is, the self-scanning type light-emitting array as described above cannot be suitably applied to an optical printer which forms printing image by light emission of pixels. That is, elements to emit light and those not to emit light in the light-emitting array cannot be set in accordance with a printing image. A series of light emission required for self-scanning of the light-emitting array appears as a fog on the printing surface and degrades the resolution of the printed image.
In addition, since a turn-on time of each element of the self-scanning light-emitting array is a fraction of the number of elements in a line, a sufficient light amount required for printing cannot be obtained. For example, when one line of a light-emitting array consists of 1,000 elements, a light emission time of one element is 1/1,000 of a scanning time of the line.